Apparatus for making electrographs

ABSTRACT

An electrographic apparatus in which a relative movement is established between a photosensitive element exhibiting persistent internal polarization and a corona discharge device while an optical system creates a light image through the corona discharge device, opposite polarity field charges are created in the photosensitive element in succession, and the light image is formed in the element during the second field charge whereby a latent image is created electrostatically in the photosensitive element corresponding to the projected light image.

United States Patent [1 1 Watanabe et a1.

[ Aug. 14, 1973 APPARATUS FOR MAKING ELECTROGRAPHS [75] inventors: Yoshlyukl Watanabe, Tokyo; Koichi Kinoshlta, Narashino, both of Japan [73] Assignee: Katsuragawa Denkl Kabushlka Keisha, Tokyo-to, Japan [22 Filed: Apr. 2, 1970 211 Appl. No.: 25,009

Related US. Application Data [62] Division of Ser. No. 481,365, Aug. 20, 1965, Pat. No.

[30] Foreign Application Priority Data Oct. 20, 1964 Japan 39/59570 [52] U.s. Cl. 355/3 [51] Int. Cl G03g 15/00 [58] Field 0! Search.- 355/15, 17

[56] v References Cited UNITED STATES PATENTS 3,268,331 8/1966 Harper 355/17 X 3,537,786 11/1970 Schlein 355/3 Primary Examiner-John M. l-loran Attorney-Marmorek & Bierman ABSTRACT An electrographic apparatus in which a relative movement is established between a photosensitive element exhibiting persistent internal polarization and a corona discharge device while an optical system creates a light image through the corona discharge device, opposite polarity field charges are created in the photosensitive element in succession, and the light image is formed in the element during the second field charge whereby a latent image is created electrostatically in the photosensitive element corresponding to the projected light image.

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APPARATUS FOR MAKING ELECTROGRAPHS This application is a division of our copending application Ser. No. 481,365, filed Aug. 20, 1965 now U.S. Pat. No. 3,536,483.

This invention relates to an apparatus for making electrographs, and more particularly to improvements relating to the apparatus for producing an electrostatic latent image of an object disclosed in our previous patent application Ser. No. 471,606, filed on July 13, 1965, now U.S. Pat. No. 3,457,070, of which the first mentioned application is a continuation-in-part application.

According to said patent application, use is made of a photosensitiveelement consisting of a photosensitive layer, a highly insulative film bonded to the upper surface of the photosensitive layer and an electrode bonded to the rear surface of said photosensitive layer, and an electrostatic image of a light image of an object is formed on the highly insulative film by a method comprising the steps of disposing a transparent electrode in contact against said highly insulative film, applying a first d-c voltage of one polarity across said transparent electrode and said electrode of the photosensitive element in the absence of external light rays, applying a second d-c voltage of opposite polarity immediatley after or a predetermined interval after the interruption of said first d-c voltage, projecting a light image of an object upon said photosensitive layer through said transparent electrode while said second d-c voltage of opposite polarity is being applied, interrupting the application of said second d-c voltage concurrently with or a short time after projection of said light image is stopped, and separating said transparent electrode from said photosensitive element thereby to form on said highly sensitive film an electrostatic latent image of said light image.

The method described above is more advantageous than any one of many prior methods of making electrographs including the so-called Xerography and persistent internal polarization methods, especially in that the electrostatic latent image is not released by later irradiation of light rays, so that the latent image formed can be stored and developed under light rays and can be released or erased only by applying an electric field.

However, as a result of further research we have found that by the method described above, upon interruption of the application of the voltage or external field, a substantial percentage of the electric charge is released without contributing to the intensity of the latent image. in addition, said method cannot produce the latent image continuously] It is therefore an object of the invention in our application Ser. No. 481,365 to provide an intense electrostatic latent image of resolution.

A further object of that invention is to provide a novel method of forming an electrostatic latent image which can prevent attenuation thereof before development.

A still further object of that invention is to provide a novel method of continuously forming a latent image.

An object of the present invention is to provide a new and improved apparatus for making an electrostatic latent image utilizing a corona discharge electrode in lieu of said transparent electrode.

in accordance with one aspect of this invention the same photosensitive element and the same process steps as those disclosed in said copending application Ser. No. 471,606 are used except that the transparent electrode is removed from the photosensitive element while a voltage or electric field is being applied across the transparent electrode and the electrode of the photoconductive element, thus greatly improving the resolution of the latent image.

In accordance with the aspect of the invention of application Ser. No. 48l,365 a photosensitive element is prepared by bonding finely divided particles of a photosensitive material by means of an electric insulating material which is transparent to light, shaping the bonded particles into a thin layer to provide a photosensitive layer, integrally bonding to one surface of said photosensitive layer a transparent highly insulative film and integrally applying an electrode on the opposite surface of said photosensitive layer, and an electrostatic latent image of an object is formed by the steps of positioning a corona discharge electrode in close proximity to but spaced apart by a definite distance from the surface of said highly insulative film, continuously moving said corona discharge electrode to scan said photosensitive element while a high potential of one polarity is being applied across said corona discharge electrode and said electrode of said photosensitive element, applying a second high potential of the opposite polarity across said electrodes and continuously moving said corona discharge electrode relative to said photosensitive element to scan it concurrently with the irradiation of a light image of an object onto said photosensitive element thereby to form on said photosensitive element an electrostatic latent image of said light image.

An electrographic apparatus embodying the principle of the present invention adapted to continuously form a latent image comprises: a cylindrical photosensitive element consisting of a photosensitive layer, a transparent highly insulative film integrally bonded to one surface of said photosensitive layer and an electrode of an electric conductive material integrally bonded to the opposite surface of said photosensitive layer; means to rotate said cylindrical photosensitive element; a first corona discharge electrode located on one side of said photosensitive element in close proximity to the surface of said highly insulative film, said corona discharge electrode including a high voltage discharge electrode and a cylindrical grounded electrode enclosing said high voltage electrode and a transparent electrode attached to one end of said grounded electrode opposite to said photosensitive element; a lens system to project a light image of an object on the surface of said highly insulative film whereby to continuously scan said photosensitive element; a second corona discharge electrode located in close proximity to said highly insulative film on the near side of said first corona discharge electrode is viewed in the direction of said cylindrical photosensitive element, said second corona discharge electrode including a high voltage electrode and a cylindrical grounded electrode enclosing said last mentioned high voltage electrode; a light shield means for said first and second corona discharge electrodes; and means to apply a high d-c voltage across the electrode of said photosensitive element and the high voltage electrode of said first corona discharge electrode.

The novel features which characterize the invention are set forth with particularity in the appended claims.

The invention itself, however, both as to its organization and method of operation together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawings in which like parts are designated by like reference numerals, and in which:

FIGS. 1 and 2 are fragmentary perspective views, partly broken away, of two forms of photosensitive elements suitable for use in the method of making electrographs according to this invention;

FIG. 3 is a diagrammatic representation of the arrangement of various elements at the time of forming an electrostatic latent image;

FIG. 4 is a graph to represent the relation between the electric field and irradiation of a light image;

FIG. 5 is a schemmatic equivalent circuit of the pho tosensitive element;

FIGS. 60 and 6b show graphs of current characteristic curves of a photosensitive layer at the time of voltage application;

FIG. 7 is a sectional view'of a corona discharge electrode suitable for use in carrying out this invention; and

FIGS. 8 and 9 show different arrangement of various elements wherein the corona discharge electrode shown in FIG. 7 is utilized.

As has been pointed out before, in carrying out the method aspect of the invention claimed in application Ser. No. 481,365, a photosensitive element as disclosed in our previous patent application Ser. No. 471,606 is utilized. More specifically, as shown in FIG. 1 of the accompanying drawing, the photosensitive element comprises a photosensitive layer 1 of a photoconductive material, a film 2 of highly insulative material and an electrode 3 which are bonded into a unitary structure. The photosensitive layer 1 as described in our application Ser. No. 471,606 can be of the type exhibiting persistent internal polarization, and containing relatively shallow type levels. CdS, ZnS, ZnO, CdSe, PbS, ZnSe, ZnTe, CdTe are the preferred materials. In the case shown in FIG. 2 an additional film of highly insulative material 2a is interposed between the photosensitive layer 1 and the electrode 3.

In both types of the photosensitive elements, the highly insulative film 2 is bonded to the photosensitive layer 1 by means of a highly insulative binder, and the electrode 3 is bonded by means of a suitable binder. The thickness of the photosensitive element is selected to be sufficiently thin and flexible in order to prevent decrease in the optical resolution and to form a firm bond with a transparent electrode which is described later.

As shown in FIG. 3, a transparent electrode 4 is urged against the upper surface of the highly insulative layer 2 by means of a suitable means, not shown, and a source of power supply, such as a battery 5, and a changeover switch 6 are provided in order to apply a d-c voltage of positive or negative polarity across the electrode 3 and the transparent electrode 4. Further various component elements are so arranged that the light image to be recorded can be projected or irradiated upon the photosensitive layer I thrQugh the transparent electrode 4, thus permitting suitable control of the application of voltage and irradiation of the light image. A

According to said previous patent application Ser. No. 471,606 the method of forming an electrostatic image comprises the steps of (a) urging the transparent electrode 4 against the surface of the highly insulative film 2 of the photosensitive element at a time to FIG. 4, (b) applying a d-c field of a suitable polarity which is selected according to the polarity desired for the latent image between the electrode 3 and the transparent electrode 4 during the period of from t, to in the absence of irradiation of light rays, (c) reversing the polarity of the applied field at a time I, while at the same time projecting the light image upon the photosensitive layer 1 through the transparent electrode 4 and the highly insulative film, (d) concurrently interrupting the application of the electric field and the projection of the light image at a time and (e) separating the transparent electrode 4 from the photosensitive element after a suitable time interval or at a time t thereby to fonn an electrostatic latent image of the light image on the surface of the photosensitive element 1 wherein at bright portions of the light. image an electrostatic charge is formed having the same polarity as that of the d-c voltage applied upon the transparent electrode 4 during the interval of from t, to t;,, whereas no charge is formed at dark portions of the light image, said latent image being characterized by being not readily released or erased after separation of the transparent electrode from the photosensitive element by any external light rays but released or attenuated by the application of an electric field for the purpose of erasing it or by urging against or bring close to the surface containing the latent image a substance which is maintained at the same potential as the electrode 3 of the photosensitive element.

Thus, it is possible to render visible the latent image at any desired time by utilizing charged or not charged powder, and the visible image of the powder can be readily transfer printed onto any other suitable recording paper, if desired. The photosensitive element can be used repeatedly to produce electrostatic latent images of any desired light images by repeating the above described process steps.

However, it is difficult to apply said method of mak ing electrographs of a mechanism operating continuously at a high speed.

The principle of forming the latent image by an electrographic method employing a perfectly insulated photosensitive element is based on the fact that the distribution of the electrostatic field or electric potential applied from the outside is determined by the relative magnitudes of the impedance Z,, FIG. 5, formed by the highly insulative film 2 of the photosensitive element and a series impedance Z, formed by the layer of the photoconductive substance 1, and hence the electrostatic charge on the surface of the photosensitive element 1 is determined by the intensity of electric field across the impedance 2,.

In addition, the intensity of the electrostatic charge is primarily determined by the dielectric constants and thickness of the highly insulative film 2 and the layer of photoconductive substance 1, respectively, when the effects of air gaps caused by incomplete contact of the electrodes and the internal polarization caused by uneven distribution of charge carriers are neglected.

The dielectric constant e of the layer of the photoconductive substance is low in the absence of irradiation of light rays from the outside, so that the effective thickness of said layer d/e where d represents the thickness thereof, is large. On the other hand, upon irradiation of light rays the dielectric constant decreases the effective thickness d/e thus resulting in an increase in the effective voltage impressed across the impedance Z as well as the intensity in the charge on the surface of the photosensitive element.

When said air gap or contact resistance at the interface between the electrode and the highly insulative film is neglected, the intensity of the electric field impressed across the highly insulative film or the field directly influencing image formation is given by the following equation.

' l l)/( 1/ 1)+( 2/ 2) 1] E 2/( 2 1 1 2) where E represents the electric field impressed upon the photo sensitive element, d and d are the thicknesses of the highly insulated film and the layer of the photoconductive layer, and e, and 6 are the dielectric constants of the film and layer, respectively. If it is assumed that d d and e, are constant, then if d is far greater than d,, the intensity of the electric field will be mainly determined by the value of 6 thus increasing with the increase of 6 However, even when d has a magnitude that cannot be neglected with respect to d it is possible to design the apparatus so as to cause sufficient variation in the intensity of the electric field that has an influence upon the image formation in response to the variation in 6 caused by external light rays.

More particularly, if 6 is increased to cause the layer of the photoconductive substance to become a perfect conductor, the intensity of the electric field impressed across the highly insulative layer will become E/d,. However, in practice, the variation of the dielectric constant of the layer of the photoconductive substance of a photosensitive element is limited by its inherent characteristics, so that, in view of the sensitivity required for the electrographic method, desirable conditions for forming satisfactory images are to establish a proper relationship between the impedances of the highly insulative film and of the layer of photoconductive layer and to limit as far as possible the increase in the dielectric constant e, of the photosensitive layer at dark portions thereof when it is irradiated by a light image in order to improve the signal-to-noise ratio, rather than to increase 6 by irradiating with intense light rays.

For this reason, in the basic method disclosed in said copending patent application Ser. No. 471,606, it is possible to decrease sufficiently the dielectric constant 5 of the layer of the photoconductive substance by applying an electric field of a polarity opposite to that of the electric field which is applied in a dark space for the purpose of forming an image prior to the irradiation by the light image.

We have analyzed the effect of the electric field of the opposite polarity which is applied prior to the irradiation by light rays and the effect of the electric field which is applied concurrently with the irradiation by light rays upon the formation of the latent image and have found many facts as described below. In order to facilitate understanding the step of applying a d-c field in the dark space at the first stage is hereinafter referred to as application of a first electric field or field, and the step of applying a d-c field of the opposite polarity relative to that of the first field concurrently with the irradiation of the light image is referred to as the application of a second electric field.

Application of the first field results in the erasure of the hysteresis caused by irradiation of light rays upon the photosensitive element or the hysteresis of the electrostatic charge, so that a signal-to-noise ratio is obtained upon subsequent irradiation of the light image. Although such a phenomenon is observed in any photosensitive element utilizing one of many photoconductors, its effects are not definite but vary with the type of photoconductor used. It was found that photoconductors exhibiting remarkable effects are those having a photoconductivity closely related to the trap level, such as CdS and ZnSe. As hereinafter described CdS has both shallow and deep trap levels while ZnCdS has only deep trap levels, the former requiring post exposure to improve the latent image.

While the effect of the first field is a function of the intensity of the applied field and the interval of field applcation and varies in accordance with the state of the applied field, such an effect generally approaches a constant value within a relatively short time, for example, in less than 1/100 second in the case of a thin layer of CdS: Cu having a thickness of microns. Furthermore, the effect of increasing the intensity of the applied electric field is not constant. Thus, with a layer of CdS: Cu having a thickness of 100 microns, ample effect can be observed under a relatively weak field of less than 1,000V, and any rapid charge in said effect can be observed even when the intensity of the electric field is increased beyond 1,000V.

Since the purpose of applying the first field is to improve the signal-to-noise ratio when the light image is projected concurrently with subsequent application of the second field, it will be obvious to those skilled in'the art that any means exhibiting analogous effect may be substituted for the first field. When infrared rays having a longer wavelength than the absorption wavelength of the photoconductor are projected the signal-to-noise ratio is improved by a certain degree. However, it was found that irradiation by such infrared rays is not desirable because its effect is not comparable with that of the first field, and because there are problems concerning the interval of irradiation as well as the intensity of the irradiating light rays.

In the photosensitive layer, when the dark space re sistance thereof has been increased by the application of the first field, the dielectric constant at portions of the layer which are irradiated during subsequent irradiation of the light image is increased. Consequently. where an insulated type photosensitive element is used, as a result of the second field which is applied concurrently, the intensity of the field across portions of the highly insulative layer subjected to light rays becomes much higher than that of portions not subjected to light rays, thereby establishing a strong electrostatic charge on the upper surface of the film at portions irradiated by light rays.

It is to be understood that the electrostatic charge on the rear surface of the highly insulative film has an important function of preserving the electrostatic charge on the upper surface. However, in the electrograph technique utilizing a photosensitive layer consisting of a powder of crystals as in this invention or in those uti Iizing an element including a multiple layer construction made up by layers of different substances, it is considered that an electrostatic charge of the opposite polarity relative to that of the electrostatic charge on the surface of the element is created on the rear surface of the highly insulative film on the surface of the photosensitive layer and in the interior thereof when an electrostatic charge exists on the surface of the photosensitive element. The presence of this internal electrostatic charge is very important not only to effectively decrease the thickness of the insulative layer but also to preserve the latent image formed in a dark space. Since this function of preserving the latent image is highly effective, when the latent image is once formed, it is not released through the transparent electrode even when it is maintained in contact with the photosensitive element in the dark space.

The intensity of the electrostatic charge on the rear surface essentially depends on the quantity of the charge carriers flowing through the photoconductive layer as well as the quantity of the charge carriers flowing out from the crystals of the photoconductive substance, it being understood that flow of the photoelectric current is closely related to the characteristics of the surface of the crystal of the photoconductive substance.

More particularly, as one example, fine grains of crystals were bonded together by means of a binder having a relatively high resistance into a thin layer, a d-c voltage was applied across the layer, and the photoelectric current flowing through an external measuring circuit was measured.

Referring now to FIG. 6a which shows the results of this measurement, the response or the buildup of the photoelectric current at a time t at which irradiation with light rays of constant intensity was commenced is poor, whereas the response or attenuation of the photoelectric current at a time t at which the irradiation of of light rays was interrupted is very steep. It appears that this is caused by the resistance charge owing to variation in the difference of work functions at the interface between photoconductive crystals and the binder and by the effect of the dielectric polarization related to trap levels.

It was found that, when the resistance is measured by means of an a-c current to measure the density of current conductive electrons in the crystals of the photosensitive substance, the build-up characteristics of the photoelectric current at the commencement of irradiation of light rays is steep, whereas the attenuation is slow,contrary to the case of employing a d-c current. This proves the correctness of said assumption.

Therefore, formation of an image by the irradiation of light rays under a d-c field is effected more advantageously where all of the electrons existing in the conductive band contribute to form the image owing to rapid decrease in the resistance at the interface which occurs concurrently with the irradiation of light rays even though the build-up of the photoelectric current is slow when compared with the case wherein the electrostatic charge in the photosensitive layer or on the rear surface of the insulative film is rapidly established to contribute to the formation of the latent image as in the case of a rapid build-up of the photoelectric current. However, it is difficult to obtain satisfactory results when an attempt is made to form a latent image by applying an electric field immediately after interruption of the irradiation of light rays by ignoring the fact that numerous electrons are still remaining in the conductive band.

It has been considered that the electrostatic charge under an electric field has an intensity sufficient to balance a field applied from the outside whether the electrostatic charge is formed on the surface or whether it is formed in the interior of a photosensitive element so that, upon interruption of the application of the external field, a portion of the charge will be released.

The method of making electrographs disclosed in said previous application Ser. No. 471,606 utilizes the possibility of preserving a sufficiently strong latent image on the surface of a photosensitive element even after interruption of the external field with a transparent electrode maintained in contact with the photosensitive element since a sufficiently strong electrostatic charge remains to form the latent image, thereby maintaining a balance between the interior and exterior of the photosensitive element. However, by later investigations we have found that, upon interruption of the external field, a substantial percentage of the electric charge is released without contributing to the intensity of the latent image.

In view of variouscharacteristics described hereinbefore, this invention contemplates effective promotion of the function of preserving the image caused by the electrostatic charge on-the rear surface by separating the transparent electrode from the photosensitive element while the application of the second .field is continued after interruption of the irradiation by the light image.

In order to indicate still more fully the nature and details of the invention, the following examples are set forth.

EXAMPLE 1.

Fine particles of CdS having a mean grain size of 10 microns and activated by copper were bonded together by means of a binder consisting of cellulose nitrate and shaped into a thin layer of microns thickness. As shown in FIG. 1, to one surface of a layer of a photoconductive substance 1 consisting of said layer, a highly insulative film 2 made of a transparent polyester resin of 12.5 microns thick, for example, was cemented by means of a binder of a polyester resin. A thin layer of conductive material such as aluminum foil was cemented to the opposite surface of the layer 1 to form an electrode 3 by means of a suitable binder, thus completing a photosensitive element.

As shown in FIG. 3, an electrode 4 made of glass which is transparent to light rays, such as those sold by the Corning Glass Co., U.S.A., under the trade name of NESA GLASS, was placed upon the highly insulative film 2 and urged thereagainst by means of a suitable pressure applying device, not shown. Thereafter a d-c field or voltage of 1,5OOV was applied for 0.1 sec. with the transparent electrode 4 connected to the positive terminal and the electrode 3 of the photosensitive element connected to the negative terminal in the absence of any incident light ray. Then the polarity of the impressed d-c field was reversed concurrently with irradiation by a light image of a brightness of 20 luxes at its bright portions. After an elapse of 0.1 sec., the irradiation of the light image was stopped, and the transparent electrode 4 was separated from the photosensitive element while the application of the d-c field was continued.

After complete separation, an intense visible image was produced by first exposing the latent image to illumination by light, forinstance daylight, and thereafter developing by means of charged particles ordinarily employed in developing electrographs.

We found that the intensity of the electrostatic .charge of the latent image at portions thereof subjected to light ray irradiation reached -l ,400V, whereas those at portions not subjected to light ray irradiation reached 10OV. It was also found that the intensity of the electrostatic charge of the latent image is much higher than that obtainable when the photosensitive element is separated after interruption of the field, and that said intensity corresponds to that obtainable when 2,000V of d-c voltage is applied, the result being an improvement of more than 40 percent. In addition, latent images formed by the process of the invention are characterized by their being defined sharply or providing higher resolution. Further, in the method in which the transparent electrode 4 is separated from the photosensitive element after interruption of the application of the voltage, it is necessary to maintain the two electrodes in an insulated condition, whereas by the method of this invention this problem can be readily solved. It was also observed that the method of separating the transparent electrode from the photosensitive element while the field is being applied is also applicable with advantages to the case where the first field is applied.

The same satisfactory result was obtained by repeating the steps described hereinabove with the exception that a four-layered photosensitive element, as shown in FIG. 2, was used wherein an additional film 2a of highly insulative substance, such as a film of a polyester resin having a thickness of 12.5 microns, was inserted between the photosensitive layer 1 and the metal electrode 3.

Since the invention of application Ser. No. 481,365 relates to an improvement of the method disclosed in our previous application Ser. No. 471,606, it is advantageous to use a photosensitive element of the same type. More particularly, the photosensitive layer 1 is comprised of fine particles of a photoconductive substance which are bounded together by a binder consisting of a transparent high molecular material having a volume resistivity of more than 10' ohm-cm'and are formed into a thin layer of a thickness of less than 200 microns. On one surface of the photosensitive layer 1, there is .unitarily cemented a highly insulative film made of a transparent substance having a volume resistivity of more than l ohm-cm, a surface resistivity of ohm-cm, a thickness of less than 50 microns, and a surface-to-surface resistivity per unit area of more than 10 ohms, by means of a binder having a volume resistivity of more than 10 ohm-cm. An electrode 3 is 'unitarily cemented to the surface of the photosensitive layer 11 opposite to the side to which the highly insulative film is cemented.

Alternatively, as shown in FIG. 2, the photosensitive element may include an additional highly insulative film 2a having the same physical and electrical characteristics as the film 2 between the photosensitive layer 1 and the electrode 3 so as to decrease to less than onehalf the electric field impressed across the photosensitive layer 1 at portions thereof subjected to irradiation of light rays when a d-c field is applied across the photosensitive element.

Further, we have made detailed investigations with reference to the air gap defined between the photosensitive element and the transparent electrode pressed thereagainst and have reached the following conclusions.

As stated hereinbefore, the principle of the method of making electrographs of the invention as claimed in application Ser. No. 481,365 is based on the variation in the field intensity, which is calculated by the following equation.

E e le d, 11

If the air gap betwen the transparent electrode and the photosensitive element is large, the dielectric constant of the air layer will be equal to approximately unit, unity, each of the elements constituting the photosensitive element has substantially a higher dielectric constant, so that the field applied across the two electrodes of the photosensitive assembly will be consumed by the air gap. Accordingly, it will be clear that the effect caused by a partial charge in the interior of the photosensitive layer due to the irradiation of the light image upon the state of the electrostatic charge on the surface of the photosensitive element is not great enough to cause a great charge in that state.

Results of our experiments have recreated that, in the above described Example 1, if the air gap has a width of 50 microns, the image forming property will be greatly decreased, so that when the applied d-c voltage is increased from 1,500V to 2,000V, the potential of the electrostatic charge at portions corresponding to bright portions of the light image will be -300V, whereas that of the electrostatic charge at portions corresponding to dark portions of the light image will be 50V. It was also found that further increase of the air gap to 200 microns results in a scale-like pattern of electrostatic charge which is quite independent of the light image and is, therefore, useless for electrography.

The above described fact has already been observed in the art prior to this invention. In fact, the air gap between adjacent electrodes is strictly specified in U. S. Pat. No. 2,825,814, for example. It has been the general practice to abstain strictly from establishing a large air gap in an electric field in methods of making an electrograph by varying the distribution of an electric field applied from the outside in response to the variation in a layer of a photoconductive substance.

However, where it is desired to continuously produce latent images at a high speed by utilizing a continuous optical scanning method, it is highly desirable to apply the electric field by means of an electrode which is amply separated in order to prevent mechanical trouble of the device and mechanical stimulation of the surface of the element and to simplify the mechanism. We have succeeded in using a corona discharge electrode whereby to fulfil the above mentioned requirement, simplify the mechanism, and prolong the useful life of the photoelectric element. The following example, relates to the use of such a corona discharge electrode.

EXAMPLE 2.

The corona discharge electrode 10 utilized in these examples is shown in FIG. 7 and comprises a high voltage discharge electrode 7 in the form of a wire grid which is enclosed by a cylindrical grounded (earthed) electrode 8 with its lower end opened. The upper end of the grounded electrode may be closed by a transparent conductor 9, if desired.

As shown in FIG. 8, the corona discharge electrode 10 is disposed above the highly insulative film 2 of a horizontally disposed photosensitive element which is identical to that shown in FIG. 1. A predetermined spacing is maintained between the corona discharge electrode and the highly insulative film so that the former (10) can scan the latter along parallel paths. in front of the corona discharge electrode 10, there is arranged a light shielding plate 11 adapted to shield out the projection of the light images in the area in front of the corona discharge electrode as viewed in the direction of scanning. Both of the grounded electrode 8 and the electrode 3 of the photosensitive element are suitably grounded while the high voltage discharge electrode 7 is connected to a high voltage d-c source through a suitable polarity reversing switch, not shown.

In this manner, a positive potential was applied to the high-voltage discharge electrode 7 of the corona discharge electrode 10 in the absence of any external light rays, and the entire surface of the photosensitive ele ment was scanned at a speed of 300 mm per second. Then the element was again scanned at the same speed while a negative potential was applied to the high volt-- age discharge electrodes 7, and the light image was projected upon the photosensitive element through the transparent electrode 9.

vUpon completion of the scanning of the entire surface, the supply of the potential was interruped, and the whole system was exposed to light, for instance daylight. Such post-exposure with the photosensitive layer being ZnCdSzAg having only deep trap levels is not necessary. However, with materials such as CdSzCu having shallow and deep trap levels post-exposure is essential to improve the quality of the latent image. Such phenomena associated with post-exposure is described in detail from lines 54, column 13 through lines 50, column [4 of U. S. Pat. No. 3,457,070 issued on our application Ser. No. 471,606. Thereafter, the latent image formed on the surface of the photosenstive element was developed by means of a developer containing a toner which is commonly used in electrography and charged with positive electricity, whereupon an intense visible image wherein the toner adhered only to portions corresponding to bright portions of the light image was ob- -l,200V, whereas that of the portions corresponding to the dark portions of the light image was -l00V.

Upon further analysis of the characteristics of the device discussed in this example, the following observations were made.

a. When the thickness of the highly insulative film on the surface of the photosensitive element is reduced extremely, and the thickness of the photosensitive layer is maintained constant, the photosensitivity of the latent image is increased to improve the resolution of the latent image, but when exposed to intense daylight, the latent image has a tendency to be disturbed, and the potential of the electrostatic charge decreases slightly.

b. When an electric field dueto corona discharge is applied after irradiation by the light image, it is difficult to obtain satisfactory results owing to the rapid attenuation of the effect of the irradiation by the light image.

c. Where the same photosensitive element is used for examples land 2, the latter or the method utilizing the corona discharge electrode provides higher sensitivity even with less quantity of incident lightrays.

dJWhen the corona discharge of the second field which was commenced concurrently with the irradiation by the light image is continued after interruption thereof, the ratio between brightness and darkness of the latent image decreases.

e. Reversal of the polarities of the first and second fields does not result in substantial charge except in the case of reversal of the polarity of the latent image.

f. The relationship between the time interval of applying the first field and its effect and that between the time interval of applying the second field and its effect are identical to that obtained by utilizing a transparent electrode in Example 1, and application'time of a fraction of one second or less is satisfactory.

g. With regard to other characteristics, it may be considered that the transparent electrode is replaced by an air layer in which a corona discharge is occurring so that the air layer acts as an electrode.

Upon the basis of the above described Example 2 we have completed a novel continuous image forming mechanism utilizing an optical scanning system.

' EXAMPLE 3.

FIG. 9 represents this continuous image forming mechanism comprising a rotary drum 12 which is made of a suitable conductive material and is horizontally joumaled in a supporting frame, not shown. A photosensitive element identical to that used in Example 1 is wrapped around the rotary drum 12, and a corona discharge electrode 10 identical to that described in connection with FIG. 7 is mounted to face one side of the rotary drum with a sufficient air gap between the electrode l0 and the photosensitive element. Somewhat above the corona discharge electrode 10 there is positioned a conventional corona discharge electrode 13 with a sufficient gap between it and the photosensitive element.

High potentials of suitable polarities are respectively applied to these corona discharge electrodes 10 and 13 from a suitable source of supply via switches, if required, and appropriate elements of these discharge electrodes are grounded in a manner already described. A lens 15 is positioned on the outside of the corona discharge electrode 10 to project the light image of an ob ject 14. The lens 15 and the discharge electrode 15 are enclosed by a light shield 16. Also, the corona discharge electrode 13 and the portion of the surface of the photosensitive element located between the electrodes l0 and 13 are enclosed by a separate light shield A potential of +7,000V with respect to the electrode 3 of the photosensitive element is applied to the high potential electrode of the corona discharge electrode 13 while a potential of 7,000V is applied to the discharge electrode 7 of the corona discharge electrode 10. The rotary drum 12 is rotated in a direction indicated by the curved arrow at a peripheral speed of 300 mm/sec. while the object is moved in a direction by the straight arrow at a speed of 300 mm/sec. in synchro nism with the rotation of the drum 12, thereby projecting the light image of the object 14 at an intensity of 15 luxes through the lens 15 and the transparent conductor 9 onto the surface of the photosensitive element over a width of 20 mm. As a result, a latent image is continuously formed on the surface of the photosensitive element, having a potential of l ,200V at portions corresponding to the bright portions of the light image and lOV at portions corresponding to dark portions thereof.

A visible image is obtained by continuously developing the latent image formed by means of a charged powder commonly utilized in electrography. The visible image obtained can be transfer printed to a suitable recording paper. It was found that it is possible to use repeatedly the photosensitive element after removing the toner remaining on the surface thereof.

Then the polarities of the potential impressed upon the high voltage electrodes of the corona discharge electrodes and 13 were reversed. This is, a potential of +7,000V was applied to the corona discharge electrode 10, and a potential of 7,000V was applied to the corona discharge electrode 13 at the time of projection of the light image to repeat the above operation. As a result potentials of +1 ,200V and +100V were observed on the surface of the light sensitive element respectively at portions corresponding to bright and dark portions of the light image. This latent image can be rendered visible by utilizing a powder developer charged with electricity of the opposite polarity, and the developed image can be transfer printed.

Thus, the embodiment of the invention as claimed in application Ser. No. 481,365 provides a novel method of continuously making electrographs by utilizing a perfectly insulated photosensitive element provided with a highly insulative film on its outermost surface without relying upon any mechanical stimulation.

As will be clear from Examples 2 and 3 and descriptions relating thereto, the corona discharge utilized in this invention plays a unique role. In prior electrography arts, corona discharge has been considered as being an important factor. Since the invention of Xerography by Carlson, corona discharge devices have been actually provided for almost all practical electrographic apparatuses in order to solve the problem of imparting uniform electrostatic charge. However, in all of these prior systems which are intended to utilize directly the so-called photoconductivity, first, a uniform charge is imparted in a dark space on the entire surface of a photosensitive element, and then the charge is locally released by the subsequent irradiation of the light image.

For this reason, charging by corona discharge is im portant from the standpoint of imparting photosensitivity. In other words, since the electrostatic charge imparted to the entire surface of the photosensitive element by a corona discharge in dark space is caused to vary by irradiation of light rays, it may be said that the above described photosensitivity has the same significance as that of the photography utilizing silver salts. Therefore, if the latent image is exposed to external light rays after irradiation of the light image but prior to development, it will disappear at once.

Furthermore, it is clear that portions of the surface of the photosensitivie element that have received no or but little light rays corresponding to dark portions of the light image show higher potential of electrostatic charge.

In contrast, in this invention, since the light sensitive element is formed with a highly insulative film on its surface, the photosensitive layer contained in the element is required to operate only at the time of forming the latent image, so that the photosensitive layer is designed not to be effective after interruption of the applied field. Consequently, in spite of the fact that corona discharge is utilized, the latent image is never varied when subjected to any external light, for instance daylight, after completion of the latent image.

It is to be observed particularly that the corona discharge employed in this invention is created to produce an electric field concurrently with the irradiation by the light image, whereby to impart the electrostatic charge at portions irradiated by light rays. When compared with well known systems, this method is more effective for utilizing the applied field so that it can form the latent image at a higher sensitivity notwithstanding the fact that it utilizes a photoconductive substance as the photosensitive layer, as is apparent from above described examples. In addition, this feature of this invention suggests various applications different from conventional systems.

While the above described unique nature of photosensitivity is mainly due to the highly insulative film formed on the photosensitive element, this nature affords very advantageous conditions for the attenuation of the latent image in dark space. More specifically, in well known systems, it is essential to preserve the electrostatic charge on the surface of the layer itself of a photoconductive substance and to release rapidly this surface charge by subsequent irradiation by light rays.

Thus, in these prior systems it is necessary to resort to a suitable compromise between these two contradicting requirements. In contrast, in the photosensitive element utilized in this invention, it is only necessary to select the layer adapted to preserve the charge from any one of many highly insulative materials that are transparent to light rays employed in electrography. Thus, it is possible to select a material having high volume resistivity as well as high surface resistivity, whereby it is possible to readily form latent images which are characterized by being not sensitive to light without being accompanied by the difficulties of prior systems.

One example of systems capable of preserving latent images formed in non-photosensitive state is disclosed in U. S. Pat. No. 2,693,416, which is characterized by the steps of forming a latent image on an insulative film which is removably superimposed upon a photosensitive layer and then separating the insulative film to isolate it from the photosensitive element. This prior system also utilizes a charge in the photoconductive layer, said charge extending to the surface of the insulative layer and being caused by projecting a light image onto the insulative film which has been treated with a corona discharge in dark space. Accordingly the system has no characteristic difference in process steps over the Carlson method.

It is thus clear that the photosensitivity will be lost after separation of the insulative film and that the mechanism of forming and preserving latent images is quite opposite to that of this invention. Also, in the method disclosed in said U. S. Pat. No. 2,693,416, although means are provided to urge the separable insulative film against the layer of photoconductive substance, it is extremely difficult to assure uniform contact between different layers over a relatively wide area. In contrast, the method of the invention as claimed in application Ser. No. 481,365, wherein the electric field is applied by a corona discharge electrode positioned amply spaced from the surface of the highly insulative film which is integral with the photosensitive element, is more advantageous in that it can provide an extremely uniform latent image.

While the foregoing brief theoretical analysis has been presented for the purpose of facilitating understanding of the principle of this invention, it will be clear that the invention is not limited to this analysis.

While the invention has been shown and described in connection with some preferred embodiments thereof, it should be understood that this invention is notlimited thereto and is intended to include many modifications and alternations thereof as fall within the true spirit and scope of this invention.

For example, an electric field may be applied across the photosensitive element by any conductive electrode other than the corona discharge electrode and transparent electrode.

1. We claim:

1. An electrographic apparatus comprising a photosensitive element exhibiting persistent internal polarization and corona discharge means, means operable to create a movement between said photosensitive element and said corona discharge means;-.said corona discharge means including at least one corona discharge unit and being operable to apply opposite polarity field charges to said element in succession, an optical system for projecting a light image through said corona discharge means to said photosensitive element during the second field charge, thereby to form an electrostatic latent image corresponding to said light image on said element.

2. The electrographic apparatus according to claim 1, wherein said photosensitive element is formed as a cylinder and carried by a rotary cylinder.

3. The electrographic apparatus according to claim 1, wherein a light shield means is providedfor said corona discharge means.

4. An electrographic apparatus, as claimed in claim 1, said corona discharge means comprising a first discharge unit operable for applying a charge of one field polarity to said element, and a second corona discharge unit spaced from said first discharge unit and operative for applying a charge of opposite field polarity to said element.

5. An electrographic apparatus as claimed in claim 1, wherein said photosensitive element is selected from the group consisting of CdS, ZnS, ZnO, CdSe, PbS, ZnSe, ZnTe, CdTe.

6. An electrographic apparatus as claimed in claim 1, wherein said photosensitive element is phosphar having shallow and deep trap levels.

7. An electrographic apparatus comprising a photosensitive element consisting of a photosensitive layer exhibiting persistent internal polarization, a transparent highly insulative film integrally bonded on one surface of said photosensitive layer, and an electrode of an electric conductive material integrally bonded on the opposite surface of said photosensitive layer;

a corona discharge electrode including a high voltage discharge electrode and a cylindrical grounded electrode enclosing said high voltage electrode;

means to support said corona discharge electrode in close proximity to the surface of said highly insulative film;

means to move said corona discharge electrode across the surface of said highly insulative film to scan said photosensitive element;

means to project a light image of an object over the entire surface of said highly insulative film.

8. An electrographic apparatus comprising a cylindrical photosensitive element consisting of a photosensitive layer exhibiting persistent internal polarization, a transparent highly insulative film integrally bonded to one surface of said photosensitive layer, and an electrode of an electric conductive material integrally bonded to the opposite surface of said photosensitive layer;

means to rotate said cylindrical photosensitive element;

a first corona discharge electrode located on one side of said photosensitive element in close proximity to the surface of said highly insulative film, said corona discharge electrode including a high voltage discharge electrode and a cylindrical grounded electrode enclosing said high voltage electrode;

a lens system to project a light image of an object on the surface of said highly insulative film whereby to continuously scan said photosensitive element;

a second corona discharge electrode located in close proximity to the surface of said highly insulative film on the near side of said firstcorona discharge electrode as viewed in the direction of scanning.

9. An electrophotographic apparatus comprising a photosensitive element including a photosensitive layer exhibiting persistent internal polarization, a transparent highly insulative layer integrally bonded on one side of said photosensitive layer, and an electrode in contact with the opposite side of said photosensitive layer; corona discharge means including a high voltage dis charge electrode and a grounded electrode encircling said high voltage discharge electrode; means operable to create a relative movement between said photosensitive element and said corona discharge means, means to operate said corona discharge means to first deposit a first charge of one polarity on the surface of said highly insulative layer and then deposit a second charge of the opposite polarity on the surface of said highly insulative layer, and an optical system for projecting a light image upon said photosensitive element through said corona discharge means during the deposition of said second charge, thereby to form an electrostatic latent image corresponding to said light image von the surface of said highly insulative layer.

10. The'electrophotographic apparatus according to claim 9 wherein said corona discharge means includes a first corona discharge unit to deposit said first charge and a second corona discharge unit spaced from said first corona discharge unit in the direction of the relative movement between said corona discharge means and said photosensitive element and adapted to deposit said second charge.

t i i t 

1. We claim:
 2. The electrographic apparatus according to claim 1, wherein said photosensitive element is formed as a cylinder and carried by a rotary cylinder.
 3. The electrographic apparatus according to claim 1, wherein a light shield means is provided for said corona discharge means.
 4. An electrographic apparatus, as claimed in claim 1, said corona discharge means comprising a first discharge unit operable for applying a charge of one field polarity to said element, and a second corona discharge unit spaced from said first discharge unit and operative for applying a charge of opposite field polarity to said element.
 5. An electrographic apparatus as claimed in claim 1, wherein said photosensitive element is selected from the group consisting of CdS, ZnS, ZnO, CdSe, PbS, ZnSe, ZnTe, CdTe.
 6. An electrographic apparatus as claimed in claim 1, wherein said photosensitive element is phosphar having shallow and deep trap levels.
 7. An electrographic apparatus comprising a photosensitive element consisting of a photosensitive layer exhibiting persistent internal polarization, a transparent highly insulative film integrally bonded on one surface of said photosensitive layer, and an electrode of an electric conductive material integrally bonded on the opposite surface of said photosensitive layer; a corona discharge electrode including a high voltage discharge electrode and a cylindrical grounded electrode enclosing said high voltage electrode; means to support said corona discharge electrode in close proximity to the surface of said highly insulative film; means to move said corona discharge electrode across the surface of said highly insulative film to scan said photosensitive element; means to project a light image of an object over the entire surface of said highly insulative film.
 8. An electrographic apparatus comprising a cylindrical photosensitive element consisting of a photosensitive layer exhibiting persistent internal polarization, a transparent highly insulative film integrally bonded to one surface of said photosensitive layer, and an electrode of an electric conductive material integrally bonded to the opposite surface of said photosensitive layer; means to rotate said cylindrical photosensitive element; a first corona discharge electrode located on one side of said photosensitive element in close proximity to the surface of said highly insulative film, said corona discharge electrode including a high voltage discharge electrode and a cylindrical grounded electrode enclosing said high voltage electrode; a lens system to project a light image of an object on the surface of said highly insulative film whereby to continuously scan said photosensitive element; a second corona discharge electrode located in close proximity to the surface of said highly insulative film on the near side of said first corona discharge electrode as viewed in the direction of scanning.
 9. An electrophotographic apparatus comprising a photosensitive element including a photosensitive layer exhibiting persistent internal polarization, a transparent highly insulative layer integrally bonded on one side of said photosensitive layer, and an electrode in contact with the opposite side of said photosensitive layer; corona discharge means including a high voltage discharge electrode and a grounded electrode encircling said high voltage discharge electrode; means operable to create a relative movement between said photosensitive element and said coroNa discharge means, means to operate said corona discharge means to first deposit a first charge of one polarity on the surface of said highly insulative layer and then deposit a second charge of the opposite polarity on the surface of said highly insulative layer, and an optical system for projecting a light image upon said photosensitive element through said corona discharge means during the deposition of said second charge, thereby to form an electrostatic latent image corresponding to said light image on the surface of said highly insulative layer.
 10. The electrophotographic apparatus according to claim 9 wherein said corona discharge means includes a first corona discharge unit to deposit said first charge and a second corona discharge unit spaced from said first corona discharge unit in the direction of the relative movement between said corona discharge means and said photosensitive element and adapted to deposit said second charge. 